Electrostatic atomization of liquids

ABSTRACT

The process of atomizing liquids having conductivity in 10 6 to 10 3 ohms - centimeters range using either a negative or a positive electrostatic charge for the purpose of producing the aerosol which is deposited. In this process an ambient atmosphere is placed around the atomizing electrodes of an electric field with gases and vapors having a higher breakdown that air which vapors include short chained aliphatic compounds containing halogens and inorganic halogens. As a result, both negative and positive charges can be used in atomizing with liquids in the indicated conductivity range.

United States Patent [72] Inventors Walter Simm;

Otto Koch, both of Leverkusen, Germany [21] Appl. No. 571,243 [22] FiledAug. 9, 1966 [45] Patented Sept. 28, 1971 [73] Assignee Agia-GevaertAktiengesellschaft Leverkusen, Germany [32] Priority Oct. 15, 1965 [33]Germany [31] A 50511 [54] ELECTROSTATIC ATOMIZATION 0F LIQUIDS 1 Claim,2 Drawing Figs.

[52] U.S. Cl 239/3, 117/17.5, 117/37 LE, 117/93.4 A, 317/3 [51] Int. ClG03g 13/16, B05b 5/02 [50] Field of Search 117/37 LX, 17,93.4-93.44,17.5; 239/3, 15; 317/3; 174/127 [56] References Cited UNITED STATESPATENTS 3,206,826 9/1965 Samoden 174/127 X 2,756,368 7/1956 Gross et al.250/90 X 3,129,112 4/1964 Marvin 117/93.4 3,169,886 2/1965 Simm 117/17.5X 3,317,138 5/1967 Fraser 239/15 3,342,621 9/1967 Point et a1.. 117/173,344,992 10/1967 Norris 239/15 X FOREIGN PATENTS 901,449 7/1962 GreatBritain 1 17/37 975,717 1 H1964 Great Britain..... ll7/37 994,645 6/1965Great Britain 117/37 OTHER REFERENCES Rodine, M. T. and R. G. Herb,Effect of CCl 4 Vapor on the Dielectric Strength of Air Physical Review,Vol. 51, Mar. 15, 1937 pages 508- 511.

Corbine, James Dillon, Gaseous Conductors Theory and EngineeringApplications McGraw-Hill Book Co. Inc. New York 1941 pages 173- 177 and181-184.

Primary Examiner-William D. Martin Assistant Examiner-Edward J. CabicAttorney-Connolly & Hutz PATENIEDsEP28197l 3.808.821

INVEN'I'ORS.

WALTER SIMM, OTTO KOCH.

(lmnh w 6k Mig ELECTROSTATIC ATOMIZATION OF LIQUIDS The electrostaticatomization of liquids is already known in connection with variouslacquer spraying techniques. Among others there are those in whichlacquers are sprayed, without the use of compressed air, simply by meansof the atomizing effect of powerful inhomogeneous electric fields, andare deposited on conductive surfaces. In electrostatic lacquer sprayingon a commercial scale, one wishes to accomplish the atomization anddeposition of large quantities of lacquer economically. The ranges ofsizes of the droplets and the sign of their electric charge are ofsecondary importance. It is noteworthy that not all combinations ofpigments and solvents can be sprayed electrically and certain conditionshave to be fulfilled in respect of the electrical properties of thelacquer. These include primarily the range of electrical conductivitywhich generally has an upper limit of 10" ohm cm..

The electrostatic atomization of dye solutions and color pigmentdispersions for developing latent electrical images by theelectrophotographic process is described in German Pat. SpecificationsNos. 1,164,829, and 1,172,955. When electrically charged dye aerosolsare used for the development of images in this way, the size of thedroplets and nature of charge of the droplets are of decisiveimportance. It is thus necessary to provide special electrodearrangements, predetermined conductivity regions for relatively highelectrical conductivity of the liquids, and predetermined electricalfield distributions and field densities. This process operates within acritical field of atomizability in that the coloring liquid used in thenegatively charged aerosols must have a conductivity above 10 ohm cm..Positive atomization also becomes difficult in this conductive regionbecause interfering corona discharges then easily occur at theatomization electrodes, and may severely inhibit or even prevent theatomization.

The object of this invention is to provide a means for atomizing liquidswhich have electrical conductivity higher than l ohm cm, by the use ofboth negative and positive electrostatic charges. Another object of theinvention is the improvement of the operational safety of electrostaticatomization apparatuses.

A process has now been found in which liquids having conductivities ofup to ohm cm." or more and preferably conductivities between 10 and 10ohm cm, can be negatively or positively atomized using simple electrodesat normal pressure, and which moreover has high operational safety. Theprocess according to the invention is characterized in that the liquidsare electrostatically sprayed by atomizing electrodes which aresurrounded by an atmosphere of a gas which has a higher electricbreakdown voltage than air at an atmospheric pressure of 760 mm. Hg. Thebreakdown voltage measured with plane electrodes at a distance of 1 cm.should preferably be at least 35 KV./cm. Reference is made to Handbook,Landolt-Bornstein, Vol. IV, part 3, p. 107 stating the breakdown valueat 760 torr, C. and llg. H 0 per cubic meter of air between planeelectrodes at a distance of 1 cm. is 31.0 kv./cm.

According to a preferred embodiment of the invention, the processcomprises supplying to the air surrounding the atomization electrodes,gases or vapors which have a higher electric breakdown potential thanair. Such gases preferably contain halogen atoms in their molecules, inparticular chlorine or fluorine, and are short-chained aliphaticcompounds having preferably not more than three C-atoms, in particularthe so-called refrigerants which are used as spray propellants namelyCCI F, C,C 1 F. C Cl F cc1,i=,, CClF etc. Low boiling inorganic halogencompounds are also especially suitable in particular fluorine compoundssuch as SF, An example of a suitable vapor with high electric breakdownpotential is CC1 The formation of a corona discharge at the atomizationelectrodes is largely prevented in the presence of such gases of vaporso that a powerful electric field which does not vary with time candevelop for use in the atomization of the liquid.

The concentration of the additional gas or vapor in the atmospheric airsurrounding the atomization electrodes may vary within wide limits. Itis determined by the desired breakdown potential of the gas. atmospheresurrounding the atomization electrode. The required breakdown potentialmay depend on the electrical properties of the liquid which is to beatomized. The concentration of the gas added further depends, withincertain limits, on the degree of moisture in the air of course, and onthe breakdown potential of the gas or vapor itself. Depending on theconditions, it is usually sufficient to add quantities of about 5 to 50percent in order to achieve the breakdown potentials stated above. Ingeneral, the desired conditions are achieved by concentrations of about20 to 50 percent. There is no upper limit to the concentrations addedsince the process can, of course, be carried out particularly well inanatmosphere of the pure gas or vapor of high breakdown potential. Onthe other hand the concentrations may be limited by economicconsiderations. The average expert will find no difficulty indetermining the optimum proportion by volume of gas to add for anyparticular atomization process.

In order to carry out the process according to the invention, it is onlynecessary that the immediate surroundings of the atomization electrodeshould have a breakdown potential within the range indicated above. Theatmosphere of high breakdown potential need only be maintained up to adistance of not more than about 5 cm. from the atomization electrode.This makes it relatively easy to construct suitable apparatus for theprocess of the invention since the usual additional apparatuses whichare required for electrostatic atomization, in particular electrostaticatomization with the use of counterelectrodes or receiver electrodes forthe development of latent electrostatic images by means of dye aerosolscan be chosen just as freely as in the case of normal electrostaticatomization in air.

This process can be applied to all electrostatic atomization but isparticularly advantageous in the case of atomization of dye liquids forelectrophotographic image development. By using suitable dye solutionsand dispersions of high conductivity, it is possible to use bothnegative and positive development processes on the photoconductivelayers which normally consist of a mixture of zinc oxide and binder.Since the size of the droplets from the electrostatic dye aerosoldecreases with increasing conductivity of the liquid, it is possible toachieve by these means higher optical resolution in the development ofthe image.

Compounds which have a low chlorine content and high fluorine content,such as CCI F are particularly suitable for use as aliphaticchlorine-fluorine compounds, owing to their low toxity and generalnoninflammability. The use of sulfur hexafluoride which is also nontoxicis particularly advantageous owing to its particularly high breakdownpotential.

A suitable atomization electrode for lacquering any metal articles bymeans of electrostatic atomization is illustrated diagrammatically inFIG. 1 of the accompanying drawings. The tube ll constitutes theelectrode and preferably has sharp edges at the top and from which theliquid is atomized. This rim may be funnel-shaped, for example, andarranged as a surface of revolution about the longitudinal axis of thetube. The electrode tube 1 is surrounded by a tube 2, made for exampleof plastic, through which the additional high-breakdown gas isintroduced. Outer tube 2 has an open annular orifice 3, at the sprayingend of the electrode, through which the gas of higher breakdown voltageescapes and circulates in the immediate vicinity of the electrode rim.

A preferred means of application of the process of the invention for thedevelopment of electrostatic images is illustrated diagrammatically inthe FIG. 2 of the attached drawings. The electrophotographic layer 4which carries the outside image is attached by a support to a groundedmetal plate 5. A wire sieve 7, stretched in a metal frame 6 andconnected to a source of voltage is arranged in front of thephotoconductive layer. The frame is attached to a plastic casing 8 whichcontains the additional gas of high electric breakdown potential. Thedesired Concentration of additional gas in the electrode chamber iscontrolled by the influx rate of the gas entering the plastic casingthrough the aperture 9. Excess gas can escape through the aperture 10.The atomization electrode 11, which is fed from outside with coloreddeveloper liquid in known manner enters the plastic casing through thisaperture. The nature of the electrode itself is not important and can beof any known design.

Example 1 A metal article is coated electrostatically with a dye pigmentfrom a dispersion which has an electrical conductivity of a x10 ohm cm..The metal article is connected, for this purpose, to ground. Lacqueringis carried out by means of an atomization electrode of the type shown inFIG. I. In order to carry out the operation, the atomization electrode 1has a potential of 50 kv. with respect to ground applied to it from ahigh-voltage source. Sulfur hexafluoride is used as the additional gasand is introduced through the plastic tube 2 which surrounds theelectrode. Atomization is extraordinarily uniform, and safe in operationeven at high voltages. This makes it possible to atomize relativelylarge quantities of liquid per'unit time.

Example 2 tional gas and is introduced through the aperture 6. The

supply of this gas is controlled in such a manner that its concentrationis about 20 to 50 percent. The development liquid has the followingcomposition: 30 percent concentrated Astra-new fuchsine(Schultz-Farbstofi'tabellen, 7th Edition No. 782) 70 percent benzylalcohol. The conductivity of the dye carrying liquid is 3X10" ohm cm).The spraying time is about 10 seconds. A positive image of the negativeoriginal is obtained.

We claim:

V 1. In the process of electrostatically atomizing coloring liquidshaving an electrical conductivity in the range of 10 ohm" 1 cm. to l0ohm" cm. by an electrostatic voltage, the improvement according to whichthe coloring liquid having an electrical conductivity in said range isatomized by the field of an electrostatic charge by applying a negativeor a positive charge For atomization of the liquid in an atmospherebetween electrodes with the atmosphere within at least 5 cm. of theatomizing electrodes being of a vapor or of a gas selected from thegroup consisting of sulfur hexafluoride, dichlorodifluoromethane,trichlorofluorornethane, chlorotrifluoromethane,trichlorotrifluoroethane and dichlorotetrafluoroethane, in an amount of5 to 50 percent by volume, whereby the atomization is effectuatable byeither sign of the potential of the electrostatic atomizing electro des.

Patent 2.608.821 Dated S Inventoriis) Walter Sim 813 8-1 It is certifiedthat error appears in the aboveidentified patent and that said LettersPatent are hereby corrected as shown below:

l'lrst page, Abstract, line 2', "10 to 10 ohms centimeters" should read10" to 10" ohms cm Column 1, line 17, 1 ohm cu should read 10- ohm' cm'Column 1, line 32, "10 OM cm should read 10-5 ohmcm' Column 1, line 38,"10 ohm cm should read 10-5 ohm-l -l Column 1, line H4, "10 ohm cmshould read 10"3 Ohm" cm' Column 1, line 45, "10 and 10 ohm cm shouldread 10- and 10'' ohm' cm" Column 3, line 15, "5:10 ohm cm should read5xl0' ohm om" Column '4, line 6, "-35 k." should read -35kV.

Column lines 13-14% ,"3xl0 911E111 .om should read 32:10 ohm cm" Columnh lines 18-19, "10 ohm 1 cm to 10 ohm cm should read 10'' ohm' cm" to10" ohm" ell o Signed a nd sealed thi s 18th day of April 1972 (SEAL)Attes L:

EDWARD M.FLETCHER ,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents

